The present invention relates to a method and apparatus for the continuous production of the type of explosive where the main constituents are an aqueous solution of salts which can yield oxygen (the salt solution) and a combustible liquid which is not soluble in the salt solution.
In the production of the type of explosive known as slurry, it is normal to use various salts which can yield oxygen, together with various fuels. The salts, normally ammonium nitrate and other nitrates, are present wholly or partially as a thickened, aqueous, normally pumpable solution, and the fuels may be solid or liquid and may be soluble or insoluble in water.
It has now become a well-established practice to produce these slurry explosives in situ, by mixing the salt solution continuously with the fuel and then pumping the explosive so formed directly into the boreholes. When the fuel is a particulate material, the mixing must take place in a mixer where the material is exposed to mechanical agitation. If the fuels are pumpable, either as homogenous liquids or as liquids with particulate matter dispersed therein, the mixer can be an apparatus known as a static mixer.
There are in principle two important advantages inherent in the use of a static mixer. The first is that the mixed explosive is not exposed to any mechanical mixing which can, in various types of abnormal working conditions, lead to undesired, uncontrolled and possibly dangerous heating of the explosive. The second is that the production equipment can be built as a completely closed system from the component pumps to the hose which is lowered into the borehole for loading the hole. In this way the need for a pump for the mixed explosive is avoided, and the risk of uncontrolled heating and the risks which result from the presence of a foreign object are eliminated.
The use of liquids which are insoluble in the salt solution comes into a special category. Fuel oil is the most typical of these liquids. Although these materials are inherently easier than particulate material to meter in a closed system, it is normally impossible to obtain an adequate dispersion of the liquid in the salt solution in a static mixer.
The flow conditions in the liquid in a static mixer are normally laminar, and this is not conducive to the formation of an emulsion, especially in salt solutions with relatively high viscosity. An extended dwell time in the mixer, and the creation of a large pressure drop across the mixer are measures known to improve the formation of an emulsion of one liquid in another, but these techniques must be considered undesirable or inappropriate for the production of explosives of the type under discussion here.
It has been necessary up to now to use mechanically driven mixing means even when using liquid fuels. This has implied that the disadvantages detailed above for the mixing of explosives containing particulate matter apply also to the mixing of explosives with liquid fuels.